1. Field of the Invention
This invention relates to optical logic and memory apparatus including optical logic gates and circuits and optical flip-flops. Accordingly, it is a general object of this invention to provide new and improved apparatus, gates, circuits and flip-flops of such character.
2. General Background
Logic gates and memory elements are the basic building blocks of a digital system. A digital system can be constructed entirely from NAND (or NOR) gates and flip-flops in which the gates perform combinational logic operations and the flip-flops perform sequential logic operations and memory functions. Conventional electronic flip-flops utilize a combination of complex logic gates.
The operation of an optical AND gate has been reported by J. C. Campbell, A. G. Dentai, J. A. Copeland and W. S. Holden, "Optical AND Gate", IEEE J. Quantum Electron., QE-18, 992 (1982). Campbell et al., however, require two optical input signals at different wavelengths and an output signal at still another wavelength. Its laser delivers an output signal having an inherent pulsation period that limits the operation of the gate.
A. Lattes, H. A. Haus, F. J. Leonberger, and E. P. Ippen, "An Ultrafast All-Optical Gate," IEEE J. Quantum Electron., QE-19, 1718 (1983), propose an optical exclusive-OR circuit based on the guided wave Mach-Zehnder interferometer. Disadvantageously, the realization of this operation appears to be awaiting the discovery of materials of extraordinarily large optical non-linear coefficient.
A demonstration of optical S-R and J-K flip-flops has been reported by K. Okumura, Y. Ogawa, H. Ito, and H. Inaba, "Optical Bistability and Monolithic Logic Functions Based on Bistable-Laser Light-Emitting Diodes", 13th IQEC, paper TuBB2, Anaheim, Calif., June 1984. Their flip-flops combine many basic logic elements in the same way that electronic flip-flops are usually constructed. The speed of the complex circuitry appears extremely low (in the neighborhood of hundreds of microseconds). They are, basically, simulations of electronic flip-flops with optical input and output.
Many known bistable light emitting devices incorporate a section of saturable absorber in the laser cavity, such as, for example, H. Kawaguchi and G. Iwane, "Bistable Operation in Semiconductor Lasers with Inhomogeneous Excitation", Electron. Lett., Vol. 17, pp. 167-168 (1981); Ch. Harder, K. Y. Lau, and A. Yariv, "Bistability and Negative Resistance in Semiconductor Lasers", Appl. Phys. Lett., 40, 124 (1982); K. Y. Lau, Ch. Harder, and A. Yariv, "Dynamic Switching Characteristics of a Bistable Injection Laser", Appl. Phys. Lett., 40, 198 (1982); Ch. Harder, K. Y. Lau, and A. Yariv, "Bistability and Pulsations in cw Semiconductor Lasers with a Controlled Amount of Saturable Absorption", Appl. Phys. Lett., 39, 392 (1981); and Ch. Harder, K. Y. Lau, and A. Yariv, "Bistability and Pulsations in Semiconductor Lasers with Inhomogeneous Current Injection", IEEE J. Quantum. Electron., QE-18, 1351 (1982). As reported therein, the carrier density in the absorbing section is low when the device is in the "off" state. Switching from the "off" state to the "on" state takes tens of nanoseconds (in accordance with the immediate forementioned reference) which is the time lapse for the carrier density to build up. Such slow switching speed imposes serious limitations on the usefulness of the devices for high speed applications.